1. Field of the Invention
The present invention relates generally to the field of display systems for use in aircraft, flight simulators, and the like, and more particularly to a system which combines a generated image with an image in an observer's line-of-sight by projecting the generated image onto a cholesteric liquid crystal combiner which reflects the projected image toward the observer together with images in the line-of-sight of the observer passing through the combiner.
2. Description of the Prior Art
In aircraft and other vehicles which require nearly continuous attention to both the outside environment and to instrumentation such as control, ordinance, etc., simultaneous viewing of both is desired. To accomplish this simultaneous viewing of both the outside environment and the instrumentation, heads-up displays (hereafter referred to as HUDs) are utilized. Such a typical prior art HUD system 8 is shown in FIG. 1. Typically, HUD systems consist of an instrumentation image source 10, such as a cathode ray tube (CRT), liquid crystal display (LCD), or similar display, an image combiner 12, and optics 16 for collimating the image. The combiner is usually angled relative to the line-of-sight plane of the observer so that the projected image in the image source plane is reflected into the line-of-sight plane of the observer. The observer views the outside environment through the combiner together with the projected instrumentation image, which appears as a virtual image focussed at infinity. Thus, the instrumentation image is, in effect, superimposed on the observer's view of the outside environment.
Presently, combiners fit into one of two categories semi-reflective combiners, and holographic combiners. Semi-reflective combiners are generally composed of a body of light-transmissive material, such as glass, having flat or selectively curved faces, one such face (usually that facing the observer) being provided with a semi-reflective thin-film coating of aluminum, silver, etc.
Light incident on a semi-reflective combiner from one direction is transmitted through it, and light incident on the combiner from the opposite direction is reflected by it. However, both absolute transmission and reflection is not possible. That is, to facilitate transmission of images from the outside environment through the combiner some degree of reflectivity of the projected images by the combiner must be sacrificed, and vice-versa. For this reason, semi-reflective mirrors as combiners have relatively poor transmissivity of images from the outside environment, and low contrast of the projected images as against the images from the outside environment. Further, aluminum coatings oxidize, silver coatings tarnish, etc., so that transmissivity and reflectivity tend to decrease with age of the combiner, especially at the shorter wavelengths. A typical semi-reflective mirror combiner will, at best, transmit approximately 75% of the light from the outside environment, while reflecting approximately 25% of the light comprising the projected image to the observer.
Holographic combiners generally consist of, in addition to an image source, diffraction optics in varying complexity. The diffraction optics serve as a combiner, and typically include a layer of photosensitive organic material such as a dichromated gelatin or photographic emulsion having a diffraction grating recorded thereon. This layer is sandwiched between two layers of glass which provide structural support and protect it from physical damage. Under the principal of Bragg diffraction, the diffraction grating will diffract and reflect light in a selected bandwidth, and transmit light outside the selected bandwidth.
In operation, the holographic combiner is placed in the line-of-sight plane of an observer. All images from the outside environment in the line-of-sight plane of the observer, except for those at the diffraction/reflection wavelength, pass through the combiner. Those images at the diffraction/reflection wavelength are reflected away from the observer. A projected image at the diffraction/reflection wavelength of the diffraction grating, incident upon the combiner, is reflected in the line-of-sight plane of the observer so as to appear superimposed on the images from the outside environment.
The holographic combiner works on the principal of exposed recording media, namely utilizing the photosensitive layer. Recorded on the media is a matrix of exposed images of dots, or a grid of lines. Light incident upon the recorded images (i.e., the matrix or grid) is reflected by the holographic combiner. The light striking the holographic combiner between the recorded images passes through it undiffracted and unreflected. This implies that holographic combiners have less than absolute reflectivity. Further, light from the outside environment is filtered by the holographic combiner such as to reduce its transmission, due to the fact that the photosensitive layer is not perfectly transmissive. In effect, typical holographic combiners transmit between 70% and 80% efficiency, while reflecting projected images at between 70% and 80% efficiency.
Low transmissivity and reflectivity of the combiner is undesirable, especially in low visibility operating conditions such as at night or in inclement weather. Further, in flight simulator applications and the like it is crucial to keep the required brightness of images generated in the trainee's line-of-sight plane to a minimum in order to minimize the cost of operation and maximize the life-span of simulator image projection equipment.
Thus, there is a present need in the art for a combiner with higher transmissivity of images from the outside environment, simulator images, etc., and further with higher reflectivity of projected images such as instrumentation, etc., while maintaining the weight, complexity and cost of the optics to a minimum.